The method utilizing band division encoding is widely known as a technology capable of encoding a general acoustic signal with a little information quantity, and yet obtaining a high-quality reproduction signal. As a representative example of the encoding technique utilizing the band division, MPEG-2 AAC (Advanced Audio Coding), being an international standard technique of ISO/IEC, exists, and a wide-band stereo signal with 16 kHz or more can be encoded in a high quality at a bit rate of 96 kbps or so.
However, when the bit rate is lowered, for example, to 48 kbps or so, the band in which the high-quality audio signal can be encoded becomes equal to or less than 10 kHz or so, and the sound that subjectively feels insufficient in a high-band signal component is produced. As a method of compensating deterioration in a sound quality due to such a limit to the band, there exists, for example, the band expansion technology described in Non-patent document 1 that is called SBR (Spectral Band Replication). The similar technology is disclosed, for example, in Non-patent document 2 as well.
[Non-patent document 1] “Digital Radio Mondiale (DRM); System Specification”, ETSI, TS 101 980 V1.1.1, paragraphs 5 and 6, September 2001
[Non-patent document 2] “AES (Audio Engineering Society) Convention Paper 5553”, 112th AES Convention, May 2002
The band expansion technology, which aims at compensating a signal of a high-frequency band (high-band component) that is lost in an audio encoding process such as AAC or a band limiting process in accordance therewith, necessitates transmitting a signal (low-band component) of a band having a frequency lower than that of the band, which is compensated by the band expansion, by employing other means. Information encoded with the band expansion technology includes information that is employed for a purpose of generating a pseudo high-band component based upon the low-band component being transmitted with the other means, and adding the pseudo high-band component to the above low-band component at the time of decoding allows deterioration in a sound quality due to a limit to the band to be compensated.
The high-band component information being encoded with the band expansion technology includes energy information of the high-band component and addition signal information. The high-band energy information is calculated for each time/frequency group unit that is obtained by group-dividing a subband signal obtained by frequency-converting an input time signal in a time direction and a frequency direction, and getting a plurality of the subbands signal together. Calculating the energy information for each time/frequency group unit obtained by getting a plurality of the subbands signal together makes it possible to reduce a coding quantity to a requisite minimum. Hereinafter, the group dividing in the time direction is defined as time group generation, and the group dividing in the frequency direction as frequency group generation
The method disclosed in Patent document 1 exists as a first conventional example of the time/frequency group generation. Upon making a reference to FIG. 10, the conventional time/frequency group generation unit, which includes a time group addition unit 902 for detecting a leading edge of the signal in a signal analysis unit 900 for analyzing the signal, and adding the time group at a predetermined time period when the leading edge has been detected, and a frequency group generation unit 904 for generating the frequency group so that the group number in the frequency direction becomes few when the leading edge has been detected, outputs time group information 9003 and frequency group information 9004.
The method disclosed in non-patent document 3 exists as a second conventional example. The group-dividing scheme in the time direction is for detecting only a head of the portion in which the signal is drastically enlarged, and generating a fixed-length time group when it has been detected, to begin with the head. The group-dividing scheme in the frequency direction is for reducing the number of the groups in the vicinity of the head divided by the detection.
The method disclosed in Patent document 2 exists as a third conventional example. The group-dividing scheme in the time direction is for calculating energy of the subband signal, comparing the above energy with energy of the neighboring region in the time direction, and dividing the time group when the energy becomes equal to or more than a predetermined threshold. The group dividing scheme in the frequency direction is for performing a process similar to the process performed in the time direction in the frequency direction after generating the time group, and generating the frequency group.
[Patent document 1] JP-P2003-529787A
[Patent document 2] JP-P2006-3580A
[Non-patent document 3] “Enhanced aacPlus general audio codec; Enhanced aacPlus encoder SBR part”, 3GPP, TS 26.404 V6.0.0, September 2004